1. Field of the Invention
The invention relates to a method and an industrial furnace for using a residual protective gas as a heating gas, primarily for use in an industrial furnace such as a multichamber furnace or a pusher type furnace, which comprises a high-temperature furnace and an upstream preheating furnace. The method according to the invention may be applied to any processes and industrial furnaces where materials undergo thermal treatment using a heating gas and a protective gas.
2. Description of the Related Art
As described for example in DE 10 2008 020 449 A1, industrial furnaces have been designed in such manner that a fundamentally energy-efficient operating arrangement can be established. However, the known arrangements have been limited to the requirement that insulating covers are provided that at least partially surround the housings and are separated from the housing wall, and that convection cavities can be created between the housing wall and the insulating cover.
It would be desirable to improve energy usage efficiency of industrial heat treating furnaces by using gases such as the protective gases from the outset.
From a technical functional perspective, the endogas used in industrial heat treating furnaces may be defined as a gas mixture that is created in a generator and is used as a protective gas before an oxidation process. Such furnaces are referred to as atmosphere furnaces in contrast to “air” or “vacuum furnaces.”
In industrial atmosphere furnaces, a gas mixture that protects the component that is to undergo thermal treatment from undesirable chemical reactions may be used as a protective gas. Thus for example nitrogen is used as a protective gas to protect against oxidation and carburisation as well as decarburisation.
An endogas can serve as a protective gas from oxidation because carbon is given off. Endogas is therefore not a carbon carrier gas that is used for carburising components.
For the purposes of the invention, the residual protective gas does also include that which is referred to as endogas, a term which is commonly used in technical circles but is of limited practical application, but the invention is not limited solely to residual endogas.
Regarding industrial furnaces that work with protective gas, the technical community has already directed substantial efforts towards reducing pollutant emission values, as has been described in EP 0 282 715 for example.
But beyond this, it is also important to use the energy contained in the gases that are not consumed in the process.
In this context, processes for using the energy content in furnace flue gases that escape from industrial furnace installations are noteworthy:
According to DE 34 32 952C2 the stated object is to suggest a way in which the burners that burn fuel gases in the vicinity of the furnace openings while the furnace is being opened may be operated simply and economically. According to the invention, the furnace flue gas in this region should be cooled, compressed and stored, and then at least some of it should be forwarded as fuel gas to one or more burners in the area of the furnace openings. A suitable device for this purpose is suggested in the reference.
When flue gas is used as the fuel, the heat shields may be formed without relying on petroproducts at all.
A further improved and known method for using the energy content of furnace flue gases that escape from industrial furnace installations is described in DE 197 20 620 A1 and provides for collecting at least a portion of such gases and re-using them for heating, possibly with the admixture of an additional heating medium.
A variation of the known method consists of drawing the furnace flue gas off from at least one major escape point from the furnace and feeding it to one or more standard radiant heater tubes in the furnace with the aid of a blower while mixing another fuel substance with it, if necessary.
In this case, the flue gases may be supplied under additional pressure or even through the intake of a burn.
In industrial furnace installations that are known as being species-related, including those that comprise multiple chambers or treatment chambers, such as are also found in pusher furnace systems for example, process steps known to the Applicant are carried out as follows depending on the process gases used:
a) Gas Ignition and Burner Ignition in a Pusher Furnace                In the present case, natural gas is supplied in an assigned preheater furnace via a ring main of a high-temperature furnace, in which initially the gas supply is triggered. Then, an automatic start routine is performed in a controller, during which a leak test of the ring main is carried out.        
b) Gas Ignition and Burner Ignition in the Preheater Furnace                As in step a), here too the automatic start routine is run in the controller. If “open burners” are used here, the industrial furnace must first be brought into a safe basic state. To do this, first the furnace doors are closed and the industrial furnace is purged for example with a volume of air equal to five times the volume of the furnace through air lines of the associated burners. Then a leak test is performed on the heating gas and the natural gas lines using a leak test unit. The furnace doors are opened again before the burner is ignited. This burner and a pilot burner are then ignited. When a stable burner flame is reported, for example by ionisation monitors, the furnace doors are closed again and the preheater furnace is heated up to its operating temperature by the burner (functioning as a heating burner), and the pilot burner stays alight until the preheater furnace is switched off again.        
c) Gas Feed to the Pusher Furnace                When the temperature has risen above at least 750° C. for example, protective gas can be fed into the pusher furnace. Depending on the feed system, a protective atmosphere such as “endogas” or nitrogen/methanol is fed into the system as the carrier gas. The gas feed to the pusher furnace is properly completed when an overpressure set via a combustion excess pressure flap is reached and a target carbon level has also been established inside the furnace.        
An analysis of these process steps showed that because of the need to burn off the escaping flue gases the use of this endogas and protective gas combustion to increase the energy efficiency of an industrial furnace is in need of improvement.